Insects

Flies (Diptera) causing myiasis

The larvae of several different families of the order Diptera (flies) cause a condition termed myiasis, which can be defined as the infestation by fly larvae of living tissues or organs of vertebrates. From a parasitological point of view there are two types of myiasis-causing flies. One type of fly species are obligatory parasites which, for their complete development, need tissues of live hosts. The other category, that is, the facultatively parasitic fly species usually develop in dead, decaying animals or other decomposing organic materials (faeces, rotting vegetation, etc.) but sometimes deposit eggs or larvae on infected wounds or on mucous membranes of live hosts. Different terms are used to indicate which part of the body that has become infested: rectal, gastric, enteric or intestinal; urinary or urogenital; auricular; ophthalmic; dermal, subdermal, or cutaneous; and nasopharyngeal (Zumpt 1965; Harwood and James, 1979). Hall and Smith (1993) classify myiasis into three main groups: cutaneous, body cavity and accidental myiasis. Cutaneous myiasis may be subdivided into: (i) blood-sucking or sanguinivorous myiasis, that is, when fly larvae attach to the skin and suck blood (e.g. the Congo floor maggot Auchmeromyia); (ii) furuncular myiasis, that is, when larvae stay in the skin and make boil-like swellings, for example, the mango fly Auchmeromyia and the human bot-fly Dermatobia hominis; and (iii) creeping myiasis (larva migrans) when larvae burrow in human skin but cannot complete development in man (Hall and Smith, 1993). Body cavity myiasis may be subdivided into nasopharyngeal, auricular, pulmonary, and ophthalmomyiases, that is, when eggs or larvae are deposited in or remain in nose, sinuses and pharyngeal cavities, and in ear, lung, or eye, respectively. Examples of accidental myiases are intestinal (enteric, rectal) when larvae are ingested by accident or reach the intestine via the anus. Another type of accidental myiasis is urogenital myiasis when flies are attracted to infected tissues or dirty clothes. Depending on fly species, number of eggs or larvae deposited, site of oviposition or larviposition, and the health status of the host myiasis may range from an asymptomatic or benign conditions to severe and even fatal infestations.

Hypoderma spp. (cattle and deer warble flies)

The warble fly (family Oestridae, subfamily Hypodermatinae, genus Hypoderma) larvae normally infest cattle and deer, but occasionally horses and humans. The gravid females of the cattle warble flies (H. bovis, H. lineatum) usually glue their eggs to the hairs on the legs of cattle and wild bovines. The newly hatched larva burrows either directly into the skin or into the hair follicles. The larva then gradually works its way through the tissues and eventually reach the skin on the host's back where it makes a small breathing opening in the skin. The inflammation of the surrounding tissues causes a swelling denoted as a warble. When mature the larvae leave the host by dropping to the ground in which they pupate. The lifecycle and biology of the reindeer or caribou warble fly, Hypoderma (=Oedemagena) tarandi, is similar to that of the cattle-infesting warble flies. Hypoderma tarandi is a common parasite of reindeer and caribou in the northern parts of the Holarctic region. It is a well-known cause of human ophthalmomyiasis in, for example, Norway and Sweden.

Infestation of man by Hypoderma larvae may result from direct oviposition by gravid female flies as well as by handling cattle having the fur infested by recently hatched larvae. Creeping myiasis, skin abscesses, serious ophthalmic myiasis, and even intracerebral myiasis due to Hypoderma have been recorded in humans (Zumpt 1965; Hall and Smith 1993). In some cases, the larva can be squeezed out of an open wound, but in most instances surgery

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is necessary. The infestation of Hypoderma in abnormal hosts often causes serious tissue destruction by the larval migrations in the body (eyes, spinal cord, and brain) causing blindness, paralyses or even fatal damage.

Oestrus ovis (the sheep nasal botfly)

Oestrus ovis (family Oestridae, subfamily Oestrinae) is considered to be originally a Palaearctic species which has subsequently spread to most sheep-farming areas of the world (Kettle 1995). The main hosts of O. ovis are sheep and goats. When intending to oviposit the female sheep nostril fly normally hovers in front of the nostrils of sheep or goats. The eggs hatch immediately during the oviposition. The newly hatched larvae are 'sprayed' into the nose of the host by the hovering adult female fly. The larvae then develop in the nasal cavity and later move into the frontal and maxillary sinuses where larval development is completed. When fully mature, the larvae move forward and are sneezed out by the host and drop to the ground where they burrow into the soil and pupate. Clinical symptoms in sheep and goats may range from mild discomfort, nasal discharge or head shaking to allergic and inflammatory responses followed by secondary bacterial infection and sometimes death. Larvae may occasionally enter the brain, causing ataxia, circling, and head pressing (Wall and Shearer 1997). Infestation causing ocular myiasis, and sometimes invasion of the brain followed by death, also occurs in dogs and humans. Oestrus ovis is, however, not capable to complete its development in these 'abnormal' hosts.

Cephenemyia spp. (deer nostril flies)

The genus Cephenemyia (family Oestridae, subfamily Oestrinae) is restricted to the Holarctic, and the larvae develop only in Cervidae (deer). The developmental biology of Cephenemyia trompe (a parasite of reindeer in the Palaearctic), of C. auribarbis (a parasite of caribou in the Nearctic) and related Cephenemyia species is very similar to that of O. ovis. The female fly deposits first-stage larvae in the nostrils of the host and the larvae subsequently move to the pharyngeal and nasal cavities. The mature larvae move to the pharyngeal and nasal cavities, leave the host and pupate in the ground. Cephenemyia spp. are, like O. ovis, often the cause of ocular myiasis in humans.

Gasterophilus spp. (stomach bots of equines)

Nine species of Gasterophilus (family Oestridae, subfamily Gasterophilinae) have been recorded from equines; six species parasitize domestic horses and donkeys. Gasterophilus intestinalis is the most important and most widely distributed of the horse bot flies. It was originally a Palaearctic species, but is now common also in North America and in Australia. The Gasterophilus larvae are normally parasites of equines. In the northern temperate region there is one generation per year. Typically, the eggs are laid on the host, licked into the mouth by the host so that the first instar larvae can develop in the tissues of the oral cavity. The second and third instar larvae attach themselves to the intestinal mucosa where they feed for several months. The prepupae are expelled with the faeces and then pupariate.

Occasionally the 1-2mm long, first instar Gasterophilus larvae will penetrate the human skin and cause a creeping myiasis (cutaneous larva migrans) in which the larvae move in the epidermis up to 20mm per day, causing considerable irritation (Kettle 1995). Similar symptoms are caused by certain tropical human-parasitizing nematodes. There are records that Gasterophilus larvae can cause ocular myiasis.

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'Accidental' myiasis

Ingestion of food or liquid contaminated with living eggs or larvae of fly species belonging to a number of different families, but most often Calliphoridae (blow flies, in particular Calliphora and Lucilia), Muscidae (including the house-fly Musca domestica), and Sarcophagidae (flesh-flies), cause enteric (intestinal) myiasis. Usually, the condition passes unnoticed without any signs or symptoms. However, enteric myiasis may cause severe disturbances including malaise, abdominal pain, and diarrhoea with bloody discharge. Living or dead larvae may be present in stool or vomit.

Rectal myiasis may occur when gravid female flies are attracted to soiled clothes or skin on which they deposit eggs or larvae. The larvae then gain entrance to the intestine via the anus. A number of species in several different families of Diptera can be involved in rectal myiasis. Species most often encountered in rectal myiasis are excrement feeders such as certain species of Muscidae and Sarcophagidae, and the rat-tailed larvae of the drone fly Eristalis tenax (Syrphidae).

Many cases of wound or traumatic myiasis, in which fly larvae develop in skin wounds or lesions may be included as an accidental type of myiasis. There are, for instance, numerous reports from hospitals and similar institutions where fly larvae have been encountered in leg sores of geriatric patients. Species of flies infesting such sores usually belong to Calliphoridae, Sarcophagidae, Fanniidae, Muscidae, and Phoridae.

Myiasis-causing flies commonly imported from the tropics

Dermatologists, infectious disease specialists, general practitioners, and other physicians should be acquainted with the fact that people who have recently arrived from the subtropics or tropics can sometimes have their signs and symptoms in skin or other tissues explained by infestation of myiasis-causing fly larvae.

Dermatobia hominis (the human bot-fly, tórsalo)

This relatively large, purplish fly parasitises in the larval stage man, cattle, dogs, and other species of domestic and wild mammals, and birds. It is widely distributed in Latin America where it is an important cattle parasite. The larvae can complete their development to the pupal stage in humans. The larvae may sometimes cause a serious, painful, and lengthy (six weeks) form of myiasis. The condition can be fatal, particularly in children. In view of the lengthy developmental period this myiasis-causing species is one of the more common ones to be diagnosed in clinics in North America and northern Eurasia among people who have recently visited Latin America.

Cochliomyia hominivorax (the New World screw-worm fly)

This calliphorid, called the New World screw-worm fly, has its main area of distribution in Latin America. The adult C. hominivorax is a relatively large, bluish green fly with an orange-coloured head.

The gravid female lays her eggs in wounds and onto mucous membranes of the natural body orifices. Main hosts are cattle but there are numerous records of infestation also on humans. The larvae can cause serious tissue destruction; even fatal cases are known, for example, when the umbilicus of a newly born baby has become infested. Cochliomyia hominivorax has been eradicated by the mass-release of laboratory-reared, gamma-irradiated, sterile male flies from the southern United States and northern Mexico. Following an

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accidental introduction of C. hominivorax by infested livestock from South America to Libya the fly was eradicated recently by the use of the mass-release of sterile male flies.

Chrysomyia bezziana

Chrysomyia bezziana is an obligate agent of myiasis and a close relative of C. hominivorax. The biology of the two fly species is similar but C. bezziana, the Old World screw-worm fly, has its main area of distribution in the tropics of Africa and Asia.

Cordylobia anthropophaga (the tumbu fly)

The tumbu fly or mango fly (Cordylobia anthropophaga) is distributed in Africa, south of the Sahara. The adult blow-fly (family Calliphoridae) is yellowish and about 1cm long. The eggs are deposited in dry soil or sand or on clothes laid on the ground to dry, especially if there is a smell of urine or faeces. If a larva comes into contact with the skin of a potential host (humans, pigs, dogs, rodents, etc.) the small larva penetrates the skin, usually unnoticed, to reside in the subcutaneous tissues. Within a week, a boillike swelling develops. It is often painful and is often secondarily infected with bacteria. The fully developed larva leaves the host and pupates in the ground. Infestation of a single person by dozens of larvae at the same time is sometimes seen. In hospitals in northern countries, the infestation is relatively commonly recorded in patients who have just returned from tropical Africa. The larva can be partly forced out of the skin by covering the spiracles (breathing openings) by vaseline. Then the larva can be gently pulled out by a forceps.

Fleas (Siphonaptera)

The insect order Siphonaptera, that is, fleas, comprises about 2000 species and subspecies (Lewis 1995). The adult flea is a blood-sucking wingless ectoparasite with a laterally flattened body. Many fleas have well-developed legs adapted for jumping. Like the Diptera (flies, mosquitoes, and midges) the Siphonaptera has a so-called complete development (egg, larva, pupa, and adult). The larva is a wormlike creature without legs. Its food is organic material which it encounters in the lair or nest of its host. The pupa is enclosed in a cocoon. The fully developed flea can lie for several months inside the cocoon waiting for a potential host. This explains why people can be attacked by fleas in houses which have been standing unoccupied for a long time.

Most flea species are ectoparasites on mammals. A lesser number feed on birds. However, in contrast to the Anoplura, the fleas are not strictly host species specific. This implies that fleas sometimes can transmit microparasitic infections from one host species to a different host species. Fleas will abandon the bodies of dying or dead hosts. This is of importance in the epidemiology of plague: fleas leaving dying, plague-infected rodents will seek new hosts and will thereby increase the potential for transmission of the disease. An adult flea may live for a year, but plague-infected fleas have a reduced life-span.

In a country like Sweden, about 50 flea species have been recorded. Some of these, in particular the cat flea (Ctenocephalides fells), the dog flea (C. canis), and bird fleas (Ceratophyllus spp.) will occasionally attack man (Figure 23.1).

The human flea (Pulex irritans) is nowadays a rarity in Swedish houses and apartments. The modern homes are generally too dry and clean for fleas to thrive there. When P. irritans is found in Sweden, it is usually as a parasite of pigs or of wild burrow-dwelling mammals

Figure 23.1 The bird flea Ceratophyllus. Natural size c.2-3 mm. (Photo: T. G. T. Jaenson©.)

like the red fox (Vulpes vulpes). The human flea, however, is still a common parasite of humans in many other, less developed parts of the world. It thrives in dirty localities.

Plague

Plague or pest is a bacterial infection caused by Yersiniapestis. The infection is primarily a zoonosis among wild rodents. It is transmitted within these rodent populations by fleas of different genera including Xenopsylla. Occasionally, the infection is transmitted by fleas from the enzootic hosts to plague-susceptible hosts, for example, squirrels and prairie dogs, which may die in large numbers (epizootic plague). Urban, rat-borne plague may occur when synanthropic (peridomestic) rat species come into contact with plague-infected enzootic rodents or plague-infected epizootic rodents in or near urban areas. When the synanthropic rodents become infected, the risk for a human plague epidemic is great.

Transmission of plague bacteria to man is mainly by infected rat fleas (Xenopsylla spp.). The infection can follow after the bite of an infected flea; after having crushed an infected flea and then contaminated a wound, for example, the bite wound, with infectious material; after having crushed an infected flea between ones teeth; or by inhalation of dry flea faeces or other material containing plague bacteria. The global incidence of human plague is about 1500-5000 cases per year. Enzootic, asymptomatic foci are present in North and South America, North, West, Southern and East Africa and Madagascar, and in several countries in Asia. Human plague is usually a very serious disease. About 30-90% of untreated cases of bubonic plague are fatal. Pulmonic plague, which is nearly always fatal, occurs when the primary infection gains entry by inhalation of infectious material via the lungs.

Wherever plague in an urban or peri-urban area is diagnosed, control of fleas and rats should be carried out without any delay. Flea control is mainly done by treating flea- and rat-infested localities with suitable insecticides. Control of rats is mainly by removing potential food sources for rats, by baited traps, and by poisonous baits.

DDT is one of the compounds which is usually used for controlling epizootics and epidemics of plague. This serious, but presently relatively rare, disease is caused by a bacterium which naturally occurs in certain populations of rodents inhabiting areas where, in general, human cases of plague rarely occur. However, from these natural plague foci the infection may spread. Surveillance of the infection in the natural plague foci should therefore be carried out on a permanent, routine basis. Epidemics of plague can occur in any area of the world where the sanitary and environmental conditions favour the breeding of rats and their fleas in close association with man. To avoid human cases of plague in urban and suburban areas, surveillance and control of fleas and rodents are the main measures to rely on. The monitoring of resistance against chemical insecticides and rodenticides among flea and rodent populations, respectively, should, therefore, be carried out routinely, particularly in countries or regions where plague is enzootic. Environmental methods including the reduction of potential food sources for rodents, rodent trapping by baited traps, and poisonous baits to kill rodents are among the main methods recommended for control of domestic and peridomestic rodent populations. A moderately effective vaccine against plague is available for use by persons potentially becoming exposed to the infection, for example, people living within or near plague enzootic foci. Valuable information relevant for the control of plague, fleas, and rodents is provided in PAHO (1982) and WHO (1973, 1974, 1988a,b, 1991). In view of the detrimental effects caused by DDT in non-target organisms, the occurrence of high levels of resistance to DDT in a number of populations of plague vectors (WHO 1992), and the availability of relatively cheap and apparently less harmful, alternative chemicals, for example, deltamethrin, it is considered inappropriate to use any of the persistent organochlorine compounds in attempts to control flea vectors of plague ( Jaenson 1996).

Flea-borne typhus

Flea-borne typhus, endemic typhus, murine typhus, murine spotted fever, or rodent typhus is an infection due to Rickettsia mooseri (R. typhi) which is an asymptomatic infection among rodents in many parts of the world, particularly in subtropical and tropical areas. Human infection is usually acquired indoors when infectious flea faeces or an infected flea is crushed and rubbed into a (bite-) wound. Inhalation of infectious flea faeces can also lead to infection. Rickettsia mooseri is mainly transmitted to humans via rodent fleas, in particular Xenopsylla cheopis. The rickettsia can also be transmitted by the body louse (Pediculus humanus). Infection with R. mooseri is, in a similar manner as Q-fever (Coxiella burnetii) and the tick-borne typhuses (R. africae, R. conorii, etc.) occasionally recorded in Northern hospitals among patients who have recently returned from, for example, the Mediterranean area or Africa.

Blood-sucking bugs (Heteroptera)

Cimex lectularius (bed-bugs)

The bed-bugs belong to the family Cimicidae within the order true bugs (Heteroptera). All cimicids feed on blood only. Two species feed primarily on human hosts, that is, Cimex lectularius (mainly in temperate areas) and C. hemipterus (mainly in the tropics). A few decades ago, the bedbugs were almost eradicated from northern and western Europe, but at present they seem to be increasing in abundance. The bed-bugs undergo so-called hemimetabolous

Figure 23.2 The bed-bug Cimex lectularius. Natural size c.6 mm. (Drawing by Inga Thomasson after Smart (1948).)

development, that is, the development occurs gradually and there is no intervening pupal stage. In common with the lice (Anoplura) the body of the bed-bugs is dorso-ventrally flattened and seen from above it is more or less oval (Figure 23.2). The young bedbugs resemble the adults but are yellowish white and smaller. The adults are usually reddish brown in colour and about 6 mm long. The wings are reduced to small scales. Thus, the bedbugs cannot fly but they are capable of running quite rapidly. The human-parasitizing bed-bugs are active mainly at night, particularly between 2 and 4 in the morning when most people are in deep sleep. At 15°C they suck blood about once every week but at 25°C they feed about once every night. However, they can survive starvation for long periods. After the blood meal, the bed-bugs hide in a secluded place, for example, under mattresses, behind pictures on walls or behind loose wallpaper, in cracks and crevices in walls or on the floor, in furniture, etc. A bed-net impregnated with an insecticide, for example, permethrin, provides good protection against bed-bugs if one is forced to sleep in a bed-bug infested room. Transfer of bed-bugs from one place to another is believed to take place by the transportation of infested furniture, etc. Eradication of bed-bugs from infested houses is preferably done by thorough cleaning followed by insecticidal spraying of the infested rooms, particularly the bed-rooms. In particular, the hiding-places of the bed-bugs, where also the eggs are laid, should be treated with a suitable insecticide (Schofield and Dolling 1993). During their blood-ingestion, the bed-bugs inject saliva. The proteins in the saliva can cause allergic reactions with oedema and itching at the site of the bite. Continuous, massive attacks by bed-bugs will eventually cause symptoms due to the disturbed sleep and occasionally also anaemia (iron deficiency).

Blood-sucking lice (Anoplura)

All known species of blood-sucking lice (Anoplura) are obligate parasites spending their whole life-cycle on mammals. Transmission of lice from one host to another is usually by close inter-host contact.

Lice move slowly. They have no wings and cannot fly or jump. The development from egg to adult is of the so-called hemimetabolous type. This means that the morphology of the young, newly hatched louse very much resembles that of the older louse, only that the latter one is bigger. Thus, in contrast to so-called holometabolous insects, which have a 'complete' development with a pupal stage, the lice do not pass through a pupal stage.

The lice are dorso-ventrally flattened insects with powerful extremities each one ending with a strong, simple claw. Depending on the stage and species they range in length from 0.5 to 8mm (Ibarra 1993). The lice move about quite slowly and are usually seen clinging to the hairs of the host's fleece. They are highly host species specific and adapted to the normal skin temperature of the host. Healthy lice do not normally leave their host. If a louse is removed from its host the louse will usually die within one or two days. There are three species of Anoplura on man, that is, the crab louse or pubic louse (Pthirus pubis), the head louse (Pediculus capitis), and the clothing louse or body louse (Pediculus humanus). The three species of human lice, all have a world-wide distribution. The two species of Pediculus are also recorded from New World monkeys, gibbons, and the great apes while Pthirus pubis is only recorded from gorillas and man (Ibarra 1993; Kettle 1995). Thus, lice from, for instance, domestic animals cannot survive on humans and vice versa.

Pediculus capitis (the head louse)

Although the morphology of the head louse, P. capitis, is very similar to that of the clothing louse, their ecology and medical importance are quite different. The adult female P. capitis is about 3mm long. The male is slightly smaller and has dark transverse bands on the dorsal part of the abdomen. The general colour of the body is dirty yellowish; recently blood-fed lice can be reddish, but later the blood-fed louse becomes much darker (black-brown). Also, louse populations living in dark hair are usually darker than those living in light hair. The form of the body is elongated, more or less oval, and the three pairs of legs are, in contrast to those of the crab louse, almost identically shaped (Figure 23.3). The yellowish eggs are about 0.8mm long and are glued firmly, usually close onto the base of the hairs of the scalp. The glue is so strong that it cannot be dissolved without destroying the hair. The eggs will hatch after about a week. When the first stage nymph begins to hatch the egg fills with air

Figure 23.3 The head louse Pediculus capitis. Natural size c.1-2 mm. (Photo: T. G. T. Jaenson©.)

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and becomes glistening white. The egg-shells, sometimes also the eggs, are popularly called nits. The white, empty egg-shells are usually located further away from the base of the hairs. Since the hairs grow about 1cm per month, it is possible to roughly estimate for how long the infestation has been going on. The young louse resembles the adult louse but is smaller. It sucks blood from the skin of the scalp about five times per day and changes skin three times before being fully developed. The life-cycle from egg to egg takes usually three weeks. The maximum life-span of a female louse is about three weeks (Ibarra 1993).

Head lice infestations are common among all age groups, although most common among children, in all socio-economic classes throughout the world. During the 1970s in northern and western Europe, head louse infestations increased markedly. In North America, next to infections with common cold viruses head louse infestations may be the most common 'disease' affecting school children (Ibarra 1993). Transmission of head lice is almost invariably by direct bodily contact, usually by head to head contact. The close bodily contact among playing children in nursery and primary schools may, at least partly, explain why head louse epidemics are most commonly recorded among children about 5-10 years old. Until a head louse epidemic is detected it has usually been going on 'silent' during several weeks or months. Light infestations can occur even under good sanitary conditions. Infestations of head lice can occur even in clean hair (MPA 1999).

The only food of the head louse is blood. It will rapidly desiccate to death unless it has a regular supply of blood. Thus, a head louse away from the host's head will not survive for longer than about a day. This is one main reason why we consider head-to-head contact to be the main mode of transmission. However, since a louse may survive for about 24 h away from the host, it cannot be excluded that other means of transmission occasionally may occur, for example, via combs, hair brushes, caps, and helmets. If such things have been used by a louse-infested person during the last few hours delousing by washing, heating, or freezing is recommended. Children should be urged not to exchange caps, helmets, combs, brushes, etc. between each other. In the control of head louse epidemics, it is not necessary to clean other clothes, bedclothes, carpets, playthings, etc. (MPA 1999).

A louse infestation is usually detected due to the itching from the scalp. During blood-feeding, the louse injects saliva into the bite-wound. The saliva contains proteins which, when they have entered the bite-wound, will cause the itch. All louse infestations are not accompanied by itch. Children, who have recently been infested, and adult persons who have become desensitized, may lack the itch. They can, therefore, be important sources for transmission of lice. Itching bite-wounds may be severely scratched and produce further ulceration and secondary bacterial infections. A black snuff-like powder on the pillow or on the collar may be louse faeces and thus evidence of a head louse infestation. The diagnosis of a potential head louse infestation is best done by combing the hair very thoroughly close to the scalp by using a fine-toothed 'louse comb'. A magnifying lens and good light is necessary to reliably detect any live eggs, which are firmly attached to the base of the hair, or live lice which may become dislodged by the comb. By the method described, many lice may become dislodged and fall onto the ground undetected. Therefore, a more reliable method to be used within the family is that the potentially louse-infested person is sitting naked, on a white sheet. The debris removed by the comb, including that attached to the comb and that which has fallen onto the sheet, shall be inspected under a magnifying lens. A positive diagnosis is confirmed by finding live eggs, that is, eggs attached to the base of hairs, and/or by the presence of active lice. The presence of only eggs or egg shells on hairs is not a reliable sign of an ongoing louse infestation; the eggs may be dead. Since the eggs are usually attached close to the base of the hairs which grow about 1cm per month the location of the eggs or shells may reveal when the infestation began (MPA 1999).

The inspection including the tracing of contacts can be very time-consuming, but is necessary in order to get good results from the treatment. Parents should also be advised to inspect their children's scalp before the start of each school term and then preferably each week during the first two months of the term and thereafter, once a month (MPA 1999).

The eggs are so firmly 'cemented' to the hair that they cannot usually be removed or destroyed by ordinary louse-combs made of plastic. Therefore, the stronger, more fine-toothed steel combs are preferred. It is considered likely that complete elimination of relatively light head louse infestations can be achieved by thorough combing using a fine-toothed steel comb.

In order to achieve complete eradication, especially of dense infestations, it is likely that this can be achieved more rapidly and easily by combining the use of a fine-toothed steel comb with insecticidal treatment of the hair. The choice of insecticide depends partly on the resistance status of the louse population in the geographical area concerned. Therefore, it is necessary to get information about the sensitivity of the local louse population to the potentially preferable insecticides. There is no indication that head lice, under natural conditions, transmit any human-pathogenic viruses, bacteria, or other microparasites.

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